Depolarizer and spectroscope

ABSTRACT

A spectroscope having a high spectroscopic characteristic capable of eliminating a polarization dependence on an arbitrary polarization state of an incident light, and measuring a spectrum having a true central wavelength of the light. The spectroscope comprises: a depolarizer comprising: a first plate a thickness of which continuously changes in a direction of 45 degrees with a first optical axis; and a second plate a thickness of which continuously changes, and which is stuck on the first plate; wherein an angle between the first optical axis and a second optical axis of the second plate, is 45 degrees, and a first reduction direction of the thickness of the first plate and a second reduction direction of the thickness of the second plate is opposite to each other; and a spectroscopic device a dispersion direction of which intersects orthogonally with the first reduction direction.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a depolarizer used foreliminating a polarization dependence of a spectroscope.

[0003] 2. Description of Related Art

[0004] In general, a dispersion device used in a spectroscope has apolarization dependence. When such a light polarized in a particulardirection, as a linearly polarized light, is incident on the dispersiondevice, even if the incident light has a given energy, the dispersiondevice has a particular output characteristics in accordance with thedirection in which the incident light is polarized. A diffractiongrating is a representative example of the dispersion device which isused in the spectroscope. The diffraction grating has the polarizationdependence that is a diffraction efficiency varies with a polarizationstate of the incident light. In other words, a reflectance with thepolarized light component perpendicular to a groove cut in thediffraction grating and a reflectance with the polarized light componentparallel to the groove, are different from each other. Therefore,because the diffraction efficiency varies according to the polarizationstate of the incident light in the spectroscope using the diffractiongrating, a trouble occurs on measuring a spectroscopic characteristic ofthe incident light. In order to remove such a polarization dependence,it is necessary to provide a polarization scrambler which converts theincident light in the arbitrary polarization state, into a circularpolarized light or no polarized light.

[0005] A depolarizer is used as the polarization scrambler. An exampleof a depolarizer according to an earlier development, for example, asdisclosed in Patent No. 2,995,985, will be explained with reference toFIG. 4.

[0006] In FIG. 4, a reference numeral 2 denotes a depolarizer. Referencenumerals 2 a and 2 b denote crystal plates, respectively. The crystalplate 2 a has a thickness which continuously changes in a direction of45 deg. with an optical axis thereof. Further, the crystal plate 2 b hasa thickness which continuously changes in a direction of 45 deg. with anoptical axis thereof. A reference numeral 21 denotes the optical axis ofthe crystal plate 2 a. Further, a reference numeral 22 denotes theoptical axis of the crystal plate 2 b. The crystal plate 2 a and thecrystal plate 2 b have similar shapes to each other. The depolarizer 2is constituted by sticking the crystal plate 2 a and the crystal plate 2b so that the optical axis 21 and the optical axis 22 intersectsorthogonally with each other.

[0007] Next, an operation of the depolarizer 2 shown in FIG. 4 will beexplained with reference to FIG. 5. FIG. 5 is a side view of thedepolarizer 2.

[0008] A crystal has an optical axis extending to a particular directionon the basis of a crystalline structure. When a light enters thecrystal, the light is separated to a plane light parallel to the opticalaxis and a plane light perpendicular to the optical axis. Then, theplane lights travel in the crystal at phase speeds which are differentfrom each other, respectively. This phenomenon will be called abirefringence. In other words, the crystal has the birefringence whichcauses a phase difference between a light component oscillating in adirection parallel to the optical axis and a light component oscillatingin a direction perpendicular to the optical axis, of the light whichpasses through the crystal. The phase difference caused in the crystalis proportional to the thickness of the crystal. Because the thicknessof each of the crystal plates 2 a and 2 b varies continuously, thethickness of each of the crystal plates 2 a and 2 b is differentaccording to a point through which the light passes. As a result, thephase difference is different according to the point through which thelight passes.

[0009] More particularly, even if the polarization states of lights L1,L2 and L3 shown in FIG. 5 are equal to each other before the lights L1,L2 and L3 pass through the crystal plates 2 a and 2 b, because the phasedifferences which are caused to the lights L1, L2 and L3 in the crystalplates 2 a and 2 b are different from each other, the polarizationstates of the lights L1, L2 and L3 are different from each other afterthe lights L1, L2 and L3 pass through the crystal plates 2 a and 2 b.Therefore, it is possible that the depolarizer 2 converts thepolarization state of the light to the state wherein a large number ofpolarization states are mixed with respect to a space. In other words,the depolarizer 2 disturbs the polarization states of the lights withrespect to a space. However, the depolarizer 2 does not have an effectwith respect to the incident light which oscillates in the directionparallel or perpendicular to the optical axis. As a result, such anincident light passes through the depolarizer 2 with keeping thepolarization state before the incident light enters the depolarizer 2.

[0010] Next, an example of the depolarizer 2 to be used will beexplained with reference to FIG. 14. FIG. 14 is a view showing aconfiguration of a spectroscope which uses the depolarizer 2.

[0011] In FIG. 14, a reference numeral 3 denotes an incident slit, 4denotes a concave mirror, 5 denotes a diffraction grating, 6 denotes aconcave mirror, and 7 a denotes an outgoing slit. The depolarizer 2 ispositioned after the incident slit 3 so as to direct the optical axisthereof in a direction of 45 deg. with grooves of the diffractiongrating 5.

[0012] The depolarizer 2 makes the incident light have the state whereina large number of polarization states are mixed. When the incident lightoscillating in the direction parallel or perpendicular to the opticalaxis of the depolarizer 2, is incident on the depolarizer 2, theincident light passes through the depolarizer 2 with keeping thepolarization state before the light enters the depolarizer 2. After theincident light passes through the depolarizer 2, the incident lightenters the diffraction grating 5 with the angle of 45 deg. with groovesof the diffraction grating 5. Therefore, even if the incident light isincident on the depolarizer 2 in any polarization state, the incidentlight is incident on the diffraction grating 5 in an always constantratio between the light component oscillating in the directionperpendicular to the grooves of the diffraction grating 5 and the lightcomponent oscillating in the direction parallel to the grooves. As aresult, the diffraction efficiency does not vary in the spectroscopeaccording to the polarization state of the incident light.

[0013] Next, problems according to an earlier development will beexplained with reference to FIG. 11. FIG. 11 is a side view of thedepolarizer 2.

[0014] Because the optical axis of the crystal plate 2 a and the opticalaxis of the crystal plate 2 b intersect orthogonally with each other,the light which is parallel to the optical axis of the crystal plate 2 ais perpendicular to the optical axis of the crystal plate 2 b.Therefore, because refractive indexes are different from each other atboth sides of the inclined surface between the crystal plates 2 a and 2b, the light is refracted on the inclined surface. Furthermore, arefraction angle to the light component oscillating in the directionparallel to the optical axis 21 of the crystal plate 2 a and arefraction angle to the light component oscillating in the directionperpendicular to the optical axis 21 are different from each other.

[0015] More specifically, a light component of an incident light L4shown in FIG. 11, oscillating in the direction parallel to the opticalaxis 21 becomes a refracted light L5. Further, a light component of theincident light L4, oscillating in the direction perpendicular to theoptical axis 21 becomes a refracted light L6. In other wards, there is aproblem in which the incident light is separated into two light raysalong the direction of the inclined surfaces in depolarizer 2.

[0016] Accordingly, in also FIG. 14, the light is separated into twolight rays in the depolarizer 2. As a result, two focal point positionsare formed on the outgoing slit 7 a.

[0017]FIG. 6 is a front view of the outgoing slit 7 a shown in FIG. 14.In FIG. 6, a reference mark F2 denotes a focal point position in casethe depolarizer 2 is not provided in the spectroscope. Reference marksF1 and F3 denote focal point positions in case the depolarizer 2 isprovided in the spectroscope. $\begin{matrix}{E_{0} = {\frac{1}{\sqrt{2}}\begin{pmatrix}{\cos \quad \varphi} & {{- \sin}\quad \varphi} \\{\sin \quad \varphi} & {\cos \quad \varphi}\end{pmatrix}\begin{pmatrix}{\exp \left( \frac{{- i}\quad \delta}{2} \right)} \\{\exp \left( \frac{\delta}{2} \right)}\end{pmatrix}{\exp \left\lbrack {i\left( {{2\pi \quad {ft}} - \delta_{0}} \right)} \right\rbrack}}} & {{Eq}.\quad 1} \\{P_{\theta} = \begin{pmatrix}{\cos^{2}\theta} & {\cos \quad {\theta \cdot \sin}\quad \theta} \\{\cos \quad {\theta \cdot \sin}\quad \theta} & {\sin^{2}\theta}\end{pmatrix}} & {{Eq}.\quad 2} \\{G = \begin{pmatrix}\alpha & 0 \\0 & \beta\end{pmatrix}} & {{Eq}.\quad 3} \\{E_{1} = {{G \cdot P_{45{^\circ}} \cdot E_{0}} = {\frac{1}{\sqrt{2}}\left( {{\cos \quad \varphi}{{{\cdot \cos}\quad \frac{\delta}{2}} - {{i \cdot \sin}\quad {\varphi \cdot \sin}\quad \frac{\delta}{2}}}} \right)\begin{pmatrix}\alpha \\\beta\end{pmatrix}{\exp \left\lbrack {i\left( {{2\pi \quad {ft}} - \delta_{0}} \right)} \right\rbrack}}}} & {{Eq}.\quad 4} \\{E_{2} = {{G \cdot P_{{- 45}{^\circ}} \cdot E_{0}} = {\frac{1}{\sqrt{2}}\left( {{\sin \quad \varphi}{{{\cdot \cos}\quad \frac{\delta}{2}} + {{i \cdot \cos}\quad {\varphi \cdot \sin}\quad \frac{\delta}{2}}}} \right)\begin{pmatrix}{- \alpha} \\\beta\end{pmatrix}{\exp \left\lbrack {i\left( {{2\pi \quad {ft}} - \delta_{0}} \right)} \right\rbrack}}}} & {{Eq}.\quad 5} \\{P_{1} = {{E_{1} \cdot E_{1}^{*}} = {\frac{1}{2}\left( {{\cos^{2}{\varphi \cdot \cos^{2}}\frac{\delta}{2}} + {\sin^{2}{\varphi \cdot \sin^{2}}\frac{\delta}{2}}} \right)\left( {\alpha^{2} + \beta^{2}} \right)}}} & {{Eq}.\quad 6} \\{P_{2} = {{E_{2} \cdot E_{2}^{*}} = {\frac{1}{2}\left( {{\sin^{2}{\varphi \cdot \cos^{2}}\frac{\delta}{2}} + {\cos^{2}{\varphi \cdot \sin^{2}}\frac{\delta}{2}}} \right)\left( {\alpha^{2} + \beta^{2}} \right)}}} & {{Eq}.\quad 7} \\{P = {{P_{1} + P_{2}} = {\frac{1}{2}\left( {\alpha^{2} + \beta^{2}} \right)}}} & {{Eq}.\quad 8}\end{matrix}$

[0018] Power of each of the light ray which has the focal point F1 andthe light ray which has the focal point F3 varies according to thepolarization state of the incident light. Using Jones vector notationrepresentative of the polarization state of the light, it is possible toexpress an incident light E₀ in an arbitrary completely polarizationstate as shown in Equation (1). A first component of Equation (1)represents a scalar value of a X directional component, and a secondcomponent of Equation (1) represents a scalar value of a Y directionalcomponent. In Equation (1), “f” represents a frequency, “δ₀” representsan initial phase, “δ” represents a phase difference between the Xdirectional component and the Y directional component, and “φ”represents an azimuth angle.

[0019] When the incident light represented by Equation (1) passesthrough the depolarizer 2 shown in FIG. 14, the incident light isseparated into two light rays L5 and L6. Then, the light rays L5 and L6pass through the diffraction grating 5. Two light rays which have passedthrough the diffraction grating 5 come into two focal points F1 and F3on the outgoing slit 7 a as shown in FIG. 6, respectively.

[0020] In FIG. 6, “E1” of Equation (4) represents the state of the lightray at the focal point F1, and “P1” of Equation (6) represents the powerof the light ray at the focal point F1. “E2” of Equation (5) representsthe state of the light ray at the focal point F3, and “P2” of Equation(7) represents the power of the light ray at the focal point F3. “P_(θ)”of Equation (2) represents a partial polarizer of an azimuth angle θ.“G” of Equation (3) represents a diffraction grating whose diffractionefficiency of the X directional component is equal to α and whosediffractive efficiency of the Y directional component is equal to β. “*”represents a complex conjugate in each of Equations (6) and (7). Asreadily understood from Equation (8), a total intensity of the lightrays at two focal points F1 and F3 is constant regardless of the stateof the incident light E₀. However, as readily understood from Equations(6) and (7), an intensity ratio between the light ray at the focal pointF1 and the light ray at the focal point F3 varies in accordance with thestate of the incident light E₀.

[0021] In the spectroscope shown in FIG. 14, the two light rays intowhich the light passing through the depolarizer 2 is separated, isreflected on the concave mirror 4 and diffracted by the diffractiongrating 5. Equation (0) represents a relationship between an incidentangle and a diffraction angle of the diffraction grating 5.

mλ=d cos ξ(sin ψ₁+sin ψ₂)   Eq.0

[0022] In Equation (0), “m” represents the diffraction order, “d”represents a grating constant of the diffraction grating 5, “λ”represents a wavelength of the light, “ξ” represents an angle betweenthe incident light and a surface perpendicular to grating grooves of thediffraction grating 5, “ψ₁” represents an incident angle of the incidentlight on the diffraction grating 5, and “ψ₂” represents a diffractionangle of the diffracted light by the diffraction grating 5.

[0023]FIG. 15 is a view showing a relationship of the angle ξ, theincident angle ψ₁, and the diffraction angle ψ₂. Under restriction ofpositions of the parts, there is a case that the light is reflected withdeviating from an axis of the concave mirror 4, and inputted to thediffraction grating 5 so as to be inclined in the Y axis direction. Whenthe two refracted lights L5 and L6 enter the diffraction grating 5, therefracted lights L5 and L6 are incident on the diffraction grating 5with the same incident angles ψ₁ with each other but the differentangles ξ from each other, in Equation (0). Therefore, as readilyunderstood from Equation (0), because the two lights are outputted fromthe diffraction grating 5 with the different diffraction angles ψ₂ fromeach other, there occurs displacement in two lights in X axis directionshown in each of FIGS. 14 and 15. As a result, as shown in FIG. 7, twofocal points F4 and F5 are formed in a slanting direction with thecutting direction of the outgoing slit 7 a. In other words, the focalpoints F4 and F5 are provided at the different positions in thedirection perpendicular to the cutting direction of the outgoing slit 7a.

[0024] As described above, if the focal points F4 and F5 are provided atthe different positions in the direction perpendicular to the cuttingdirection of the outgoing slit 7 a, and as explained with reference toEquations (6) and (7), the intensity ratio between the light rays at thetwo focal points F4 and F5 varies in accordance with the state of theincident light, the spectroscope outputs a measured central wavelengthwhich is different from a true central wavelength.

[0025]FIGS. 13A to 13C are views showing spectrum waveforms which areoutputted to a spectrum display unit 10 shown in FIG. 14. FIG. 13A is aview showing a measured spectrum in case the light is not separated, andone focal point is formed on the outgoing slit 7 a. FIG. 13B is a viewshowing a measured spectrum in case the intensity ratio between thelight rays at the two focal points F4 and F5 shown in FIG. 7 is equal to1:0. FIG. 13C is a view showing a measured spectrum in case theintensity ratio between the light rays at the two focal points F4 and F5shown in FIG. 7 is equal to 0:1. In FIGS. 13A to 13C, “λ₀” represents atrue central wavelength of the incident light, and “αλ” represents adifference between the true central wavelength and the measured centralwavelength. The measured spectrum obtained by the spectroscope using thedepolarizer 2 varies from the state shown in FIG. 13B to the state shownin FIG. 13C, in accordance with the polarization state of the light. Asa result, it is difficult to measure the true central wavelength of thelight.

[0026] If any one of the powers of the light rays at the focal points F4and F5 of the outgoing slit 7 a shown in FIG. 7, concerning the incidentlight in the arbitrary polarization state is always zero, and the otherof the powers is always constant, it is possible to obtain the spectrumhaving a stable central wavelength concerning the incident light in thearbitrary polarization state. For example, if it is possible to alwaysobtain the state shown in FIG. 13B, it is possible to measure thespectrum having the true central wavelength by an adjusting function ofsubtracting the constant αλ from the measured central wavelength.

SUMMARY OF THE INVENTION

[0027] The present invention was developed in view of theabove-described problems.

[0028] It is an object of the present invention to realize aspectroscope having a high spectroscopic characteristic that is capableof eliminating a polarization dependence of a spectroscopic device, onan arbitrary polarization state of an incident light, and measuring aspectrum having a true central wavelength of the light. It is anotherobject of the present invention to provide a depolarizer capable ofrealizing the above-described spectroscope.

[0029] In order to attain the above-described objects, in accordancewith an aspect of the present invention, a depolarizer (for example, adepolarizer 1) comprises: a first plate (1 b) a thickness of whichcontinuously changes in a direction of 45 degrees with a first opticalaxis (12) of the first plate; and a second plate (1 c) a thickness ofwhich continuously changes, and which is stuck on the first plate;wherein an angle between the first optical axis (12) of the first plate(1 b) and a second optical axis (13) of the second plate (1 c), is 45degrees, and a first reduction direction of the thickness of the firstplate (1 b) and a second reduction direction of the thickness of thesecond plate (1 c) is opposite to each other.

[0030] Preferably, for example, as shown in FIG. 1, a third plate (1 a)and a fourth plate (1 d) are stuck on the first plate (1 b) and thesecond plate (1 c), respectively. That is, the depolarizer comprises twoplates that are the first and second plates (1 b and 1 c ), three platesthat are the first, second and third plates (1 b, 1 c and 1 a), threeplates that are the first, second and fourth plates (1 b, 1 c and 1 d),or four plates that are the first, second, third and fourth plates (1 b,1 c, 1 a and 1 d).

[0031] Preferably, each of the first plate, the second plate, the thirdplate and the fourth plate has a birefringence crystalline structure,and is made of any one of a crystal, a calcite, a mica and a magnesiumfluoride.

[0032] As described above, as the number of plates decreases, it ispossible to manufacture the depolarizer more easily and at a lower cost.

[0033] However, as the number of plates decreases such as three platesand two plates, an optical path length difference between a thick partand a thin part of the depolarizer increases. In other words, because anaberration increases, a spot diameter on an outgoing slit of aspectroscope using the depolarizer becomes large. As a result, awavelength resolution of the spectroscope drops down. Accordingly, whenthe number of plates increases such as two plates, three plates and fourplates, the spot diameter on the outgoing slit of the spectroscopebecomes small. As a result, it is possible to improve the spectroscopein the wavelength resolution.

[0034] In accordance with another aspect of the present invention, aspectroscope comprises: the above-described depolarizer (for example,the depolarizer 1); and a spectroscopic device (diffraction grating 5) adispersion direction of which intersects orthogonally with the firstreduction direction of the thickness of the first plate (1 b).

[0035] Accordingly, because the depolarizer and the spectroscopic deviceare positioned as described above, it is possible that a direction of aninclined surface of the depolarizer, with an incident light on thedepolarizer and a direction (a groove direction in case thespectroscopic device is a diffraction grating 5) perpendicular to thedispersion direction of the spectroscopic device agree with each other.

[0036] Preferably, in the spectroscope of another aspect of the presentinvention, for example, as shown in FIG. 12, when the incident lightpassing through the depolarizer (1) is separated into first, second,third and fourth refracted lights (L7, L8, L9 and L10), any one pair ofa pair of the first and third refracted lights (L7 and L9) and a pair ofthe second and fourth refracted lights (L8 and L10) is selected and usedin a signal processing.

[0037] Preferably, in the spectroscope as described above, in order toselect any of the first, second, third and fourth refracted lights (L7,L8, L9 and L10), for example, as shown in FIG. 10, an outgoing slit isused so that any one pair of the pair of the first and third refractedlights (L7 and L9) and the pair of the second and fourth refractedlights (L8 and L10) passes therethrough, and the other pair of the pairof the first and third refracted lights (L7 and L9) and the pair of thesecond and fourth refracted lights (L8 and L10) is cut off thereby.

[0038] Preferably, the above-described spectroscope is a multi-pathspectroscope wherein a light passes through the spectroscopic device atn times, and uses the above-described depolarizer. Accordingly, it ispossible to realize the spectroscope having a high wavelength resolutionand a high wavelength accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present invention will become more fully understood from thedetailed description given hereinafter and the accompanying drawinggiven by way of illustration only, and thus are not intended as adefinition of the limits of the present invention, and wherein:

[0040]FIG. 1A is a perspective view, and FIG. 1B is an explodedperspective view, of a depolarizer 1 according to an embodiment of thepresent invention;

[0041] FIGS. 2A-F, 2A-P and 2A-S are a front view, a plan view, and aside view of a crystal plate 1 a, FIGS. 2B-F, 2B-P and 2B-S are a frontview, a plan view, and a side view of a crystal plate 1 b, FIGS. 2C-F,2C-P and 2C-S are a front view, a plan view, and a side view of acrystal plate 1 c, and FIGS. 2D-F, 2D-P and 2D-S are a front view, aplan view, and a side view of a crystal plate 1 d, of the depolarizer 1according to the embodiment of the present invention;

[0042]FIG. 3 is a view showing a configuration of a spectroscopeaccording to an embodiment of the present invention;

[0043]FIG. 4 is a view showing a configuration of a depolarizer 2according to an earlier development;

[0044]FIG. 5 is a side view of the depolarizer 2, for explaining apolarization eliminating characteristic of the depolarizer 2 accordingto an earlier development;

[0045]FIG. 6 is a front view of an outgoing slit 7 a of a spectroscopeaccording to an earlier development;

[0046]FIG. 7 is a front view of the outgoing slit 7 a of thespectroscope according to an earlier development;

[0047]FIG. 8 is a front view of an outgoing slit 7 a of the spectroscopeaccording to the embodiment of the present invention;

[0048]FIG. 9 is a front view of the outgoing slit 7 a of thespectroscope according to the embodiment of the present invention;

[0049]FIG. 10 is a front view of an outgoing slit 7 b of a spectroscopeaccording to another embodiment of the present invention;

[0050]FIG. 11 is a side view of the depolarizer 2, for explaining arefractive characteristic of the depolarizer 2 according to an earlierdevelopment;

[0051]FIG. 12 is a side view of the depolarizer 1, for explaining arefractive characteristic of the depolarizer 1 according to theembodiment of the present invention;

[0052]FIGS. 13A to 13C are views of spectrums which are displayed on aspectrum display unit 10 shown in each of FIGS. 3 and 14;

[0053]FIG. 14 is a view showing a configuration of the spectroscopeaccording to an earlier development; and

[0054]FIG. 15 is a perspective view of a diffraction grating 5 shown ineach of FIGS. 3 and 14.

PREFERRED EMBODIMENTS OF THE INVENTION

[0055] Hereinafter, a preferred embodiment of the present invention willbe explained with reference to figures, as follows. The followingdescription concerns to an embodiment of the present invention, and doesnot limit the present invention.

First Embodiment

[0056] At first, a depolarizer according to a first embodiment of thepresent invention will be explained with reference to FIGS. 1A to 2D-S.

[0057]FIG. 1A is an external perspective view of a depolarizer 1, andFIG. 1B is an exploded perspective view of the depolarizer 1. FIGS.2A-F, 2A-P and 2A-S are a front view, a plan view and a side view of acrystal plate 1 a, FIGS. 2B-F, 2B-P and 2B-S are a front view, a planview and a side view of a crystal plate 1 b, FIGS. 2C-F, 2C-P and 2C-Sare a front view, a plan view and a side view of a crystal plate 1 c,and FIGS. 2D-F, 2D-P and 2D-S are a front view, a plan view and a sideview of a crystal plate 1 d, of the depolarizer 1.

[0058] A reference numeral 1 denotes a depolarizer. Reference numerals 1a, 1 b, 1 c and 1 d denote crystal plates. Reference numerals 11, 12, 13and 14 denote optical axes of the crystal plates 1 a, 1 b, 1 c and 1 d,respectively.

[0059] The depolarizer 1 comprises a crystal plate 1 b as a first plate,a crystal plate 1 c as a second plate, a crystal plate 1 a as a thirdplate, and a crystal plate 1 d as a fourth plate. The crystal plates 1a, 1 b, 1 c and 1 d are stuck so that the depolarizer 1 has a constantthickness on the whole, as shown in FIG. 1A. The first, second, thirdand fourth plates can be made of a crystalline material having abirefringence, such as a calcite, a mica, a magnesium fluoride and soon, instead of a crystal. In addition, at least one of the crystalplates 1 a and 1 d may be omitted in the depolarizer 1. According to thefirst embodiment, the present invention will be basically explained incase the depolarizer 1 uses the crystal plates 1 a, 1 b, 1 c and 1 d.

[0060] As shown in FIGS. 1A to 2D-S, each of four crystal plates 1 a, 1b, 1 c and 1 d is formed in a shape so that one surface thereof isinclined and a thickness thereof changes continuously.

[0061] As shown in FIGS. 1A and 1B, four crystal plates 1 a, 1 b, 1 cand 1 d are superposed so that each of the crystal plates 1 a, 1 b, 1 cand 1 d compensates the thickness thereof with one of the adjacentcrystal plate. Therefore, the crystal plates 1 a and 1 b are stuck oneach other, the crystal plates 1 b and 1 c are stuck on each other, andthe crystal plates 1 c and 1 d are stuck on each other. Further, thereduction direction of the thickness of the crystal plate 1 a isopposite to the reduction direction of the thickness of the crystalplate 1 b. Similarly, the reduction direction of the thickness of thecrystal plate 1 b is opposite to the reduction direction of thethickness of the crystal plate 1 c. The reduction direction of thethickness of the crystal plate 1 c is opposite to the reductiondirection of the thickness of the crystal plate 1 d.

[0062] The thickness of the crystal plate 1 a continuously changes in adirection of 45 deg. with the optical axis 11. The thickness of thecrystal plate 1 b continuously changes in a direction of 45 deg. withthe optical axis 12. The thickness of the crystal plate 1 c continuouslychanges in a direction perpendicular to the optical axis 13. Thethickness of the crystal plate 1 d continuously changes in a directionparallel to the optical axis 14.

[0063] As readily understood from the above description, the opticalaxis 11 of the crystal plate 1 a and the optical axis 12 of the crystalplate 1 b intersect orthogonally with each other. An angle between theoptical axis 12 of the crystal plate 1 b and the optical axis 13 of thecrystal plate 1 c is 45 deg. The optical axis 13 of the crystal plate 1c and the optical axis 14 of the crystal plate 1 d intersectorthogonally with each other.

[0064] As described above, because the thickness of each of the crystalplates 1 a, 1 b, 1 c and 1 d continuously changes, the thickness or thetransparent distance of each of the crystal plates 1 a, 1 b, 1 c and 1d, through which the light passes, changes according to the positionthrough which the light passes. Therefore, because the phase differencewhich is occurred in the crystal plates 1 a, 1 b, 1 c and 1 d changesaccording to the position through which the light passes, it is possiblethat depolarizer 1 converts the polarization state of the light to thestate wherein a number of polarization states are mixed with respect toa space.

[0065] There does not occur the phase difference of the polarized lightwhich oscillates in a direction parallel or perpendicular to the opticalaxis 12, in the crystal plate 1 a and the crystal plate 1 b. However,there occurs the phase difference of the polarized light in the crystalplate 1 c and the crystal plate 1 d, or in only the crystal plate 1 c incase the crystal plate 1 d is omitted from the depolarizer 1. As aresult, the light has the state wherein a number of polarization statesare mixed.

[0066] Further, there does not occur the phase difference of thepolarized light which oscillates in a direction parallel orperpendicular to the optical axis 13, in the crystal plate 1 c and thecrystal plate 1 d. However, there occurs the phase difference of thepolarized light in the crystal plate 1 a and the crystal plate 1 b, orin only the crystal plate 1 b in case the crystal plate 1 a is omittedfrom the depolarizer 1. As a result, the light has the state wherein anumber of polarization states are mixed.

[0067] Next, a configuration of a spectroscope will be explainedaccording to a first embodiment of the present invention with referenceto FIG. 3. The present invention is not limited to a Czerny-Turner typeof spectroscope shown in FIG. 3. The present invention can be applied tovarious types of spectroscopes such as a Littrow type of spectroscopeand so on. The spectroscope according to the first embodiment uses thedepolarizer 1 according to the above-described embodiment. Further, thespectroscope according to the first embodiment comprises parts similarto the spectroscope described with reference to FIG. 14, other than thedepolarizer 1. That is, when the incident light on the spectroscope,passes through the incident slit 3 and the depolarizer 1, the light isreflected on the concave mirror 4, diffracted on the diffraction grating5, and reflected on the concave mirror 6. Then, when the light passesthrough the outgoing slit 7 a, the light is received by a lightreceiving unit 8, and processed by a signal processing unit 9.Therefore, the spectrum of the light is displayed on the spectrumdisplay unit 10.

[0068] As shown in FIG. 3, the depolarizer 1 is positioned along anoptical path after the incident slit 3. Further, the depolarizer 1 ispositioned along the optical path so that the direction in which thethickness of the crystal plate 1 b continuously changes and thedispersion direction of the diffraction grating 5 which is used as aspectroscopic device, intersect orthogonally with each other. As aresult, the direction of the inclined surface formed on each of thecrystal plates 1 a, 1 b, 1 c and 1 d is parallel to a groove directionof the diffraction grating 5. In addition, the outer surfaces of thecrystal plates 1 b and 1 c, opposite to surfaces of the crystal plates 1b and 1 c, which are stuck on each other, are directed in a directioninclined with the incident light. In other words, the outer surfaces ofthe crystal plates 1 b and 1 c are two inclined surfaces which are ademarcation surface between the crystal plate 1 a (air in case thecrystal plate 1 a is omitted from the depolarizer 1) and the crystalplate 1 b, and a demarcation surface between the crystal plate 1 c andthe crystal plate 1 d (air in case the crystal plate 1 d is omitted fromthe depolarizer 1).

[0069] Next, a separation of light will be explained with reference toFIG. 12.

[0070] In the crystal plates 1 a and 1 b, because the optical axis 11 ofthe crystal plate 1 a and the optical axis 12 of the crystal plate 1 bintersect orthogonally with each other, the light oscillating in adirection parallel to the optical axis 11 of the crystal plate 1 aoscillates in a direction perpendicular to the optical axis 12 of thecrystal plate 1 b. Further, the crystalline structure having thebirefringence, has the refractive index to the light wave oscillating ina direction parallel to the optical axis and the refractive index to thelight wave oscillating in a direction perpendicular to the optical axis,which are different from each other. Therefore, because the refractiveindexes are different in both sides of the inclined surface which is thedemarcation surface between the crystal plate 1 a (air in case thecrystal plate 1 a is omitted from the depolarizer 1) and the crystalplate 1 b, the light is refracted on the demarcation surface (inclinedsurface) between the crystal plate 1 a and the crystal plate 1 b.Furthermore, the refraction angle of the light component oscillating inthe direction parallel to the optical axis 12 of the crystal plate 1 bis different from the refraction angle of the light componentoscillating in the direction perpendicular to the optical axis 12 of thecrystal plate 1 b. As a result, the light which is incident on thecrystal plate 1 b is separated into two refracted lights.

[0071] In the crystal plates 1 c and 1 d, because the optical axis 13 ofthe crystal plate 1 c and the optical axis 14 of the crystal plate 1 dintersect orthogonally with each other, the light oscillating in adirection parallel to the optical axis 13 of the crystal plate 1 coscillates in a direction perpendicular to the optical axis 14 of thecrystal plate 1 d. Further, because the refractive indexes are differentin both sides of the inclined surface which is the demarcation surfacebetween the crystal plate 1 c and the crystal plate 1 d (air in case thecrystal plate 1 d is omitted from the depolarizer 1), the light isrefracted on the demarcation surface (inclined surface) between thecrystal plate 1 c and the crystal plate 1 d. Furthermore, the refractionangle of the light component oscillating in the direction parallel tothe optical axis 13 of the crystal plate 1 c is different from therefraction angle of the light component oscillating in the directionperpendicular to the optical axis 13 of the crystal plate 1 c. As aresult, the two refracted lights which are incident on the crystal plate1 d are further separated into four refracted lights.

[0072] That is, as shown in FIG. 12, the incident light L4 is separatedinto four light rays L7, L8, L9 and L10. $\begin{matrix}{E_{0} = {\frac{1}{\sqrt{2}}\begin{pmatrix}{\cos \quad \varphi} & {{- \sin}\quad \varphi} \\{\sin \quad \varphi} & {\cos \quad \varphi}\end{pmatrix}\begin{pmatrix}{\exp \left( \frac{{- i}\quad \delta}{2} \right)} \\{\exp \left( \frac{\delta}{2} \right)}\end{pmatrix}{\exp \left\lbrack {i\left( {{2\pi \quad {ft}} - \delta_{0}} \right)} \right\rbrack}}} & {{Eq}.\quad 9} \\{P_{0} = \begin{pmatrix}{\cos^{2}\theta} & {\cos \quad {\theta \cdot \sin}\quad \theta} \\{\cos \quad {\theta \cdot \sin}\quad \theta} & {\sin^{2}\theta}\end{pmatrix}} & {{Eq}.\quad 10} \\{G = \begin{pmatrix}\alpha & 0 \\0 & \beta\end{pmatrix}} & {{Eq}.\quad 11} \\{E_{1} = {{G \cdot P_{0{^\circ}} \cdot P_{45{^\circ}} \cdot E_{0}} = {\frac{\alpha}{\sqrt{2}}\begin{pmatrix}{{\cos \quad {\varphi \cdot \cos}\quad \frac{\delta}{2}} - {{i \cdot \sin}\quad {\varphi \cdot \sin}\quad \frac{\delta}{2}}} \\0\end{pmatrix}{\exp \left\lbrack {i\left( {{2\pi \quad {ft}} - \delta_{0}} \right)} \right\rbrack}}}} & {{Eq}.\quad 12} \\{E_{2} = {{G \cdot P_{0{^\circ}} \cdot P_{{- 45}{^\circ}} \cdot E_{0}} = {{- \frac{\alpha}{\sqrt{2}}}\begin{pmatrix}{{\sin \quad {\varphi \cdot \cos}\quad \frac{\delta}{2}} + {{i \cdot \cos}\quad {\varphi \cdot \sin}\quad \frac{\delta}{2}}} \\0\end{pmatrix}{\exp \left\lbrack {i\left( {{2\pi \quad {ft}} - \delta_{0}} \right)} \right\rbrack}}}} & {{Eq}.\quad 13} \\{E_{3} = {{G \cdot P_{{- 90}{^\circ}} \cdot P_{45{^\circ}} \cdot E_{0}} = {\frac{\beta}{\sqrt{2}}\begin{pmatrix}0 \\{{\cos \quad {\varphi \cdot \cos}\quad \frac{\delta}{2}} - {{i \cdot \sin}\quad {\varphi \cdot \sin}\quad \frac{\delta}{2}}}\end{pmatrix}{\exp \left\lbrack {i\left( {{2\pi \quad {ft}} - \delta_{0}} \right)} \right\rbrack}}}} & {{Eq}.\quad 14} \\{E_{4} = {{G \cdot P_{{- 90}{^\circ}} \cdot P_{{- 45}{^\circ}} \cdot E_{0}} = {\frac{\beta}{\sqrt{2}}\begin{pmatrix}0 \\{{\sin \quad {\varphi \cdot \cos}\quad \frac{\delta}{2}} - {{i \cdot \cos}\quad {\varphi \cdot \sin}\quad \frac{\delta}{2}}}\end{pmatrix}{\exp \left\lbrack {i\left( {{2\pi \quad {ft}} - \delta_{0}} \right)} \right\rbrack}}}} & {{Eq}.\quad 15} \\{P_{1} = {{E_{1} \cdot E_{1}^{*}} = {\frac{\alpha^{2}}{2}\left( {{\cos^{2}{\varphi \cdot \cos^{2}}\frac{\delta}{2}} + {\sin^{2}{\varphi \cdot \sin^{2}}\frac{\delta}{2}}} \right)}}} & {{Eq}.\quad 16} \\{P_{2} = {{E_{2} \cdot E_{2}^{*}} = {\frac{\alpha^{2}}{2}\left( {{\sin^{2}{\varphi \cdot \cos^{2}}\frac{\delta}{2}} + {\cos^{2}{\varphi \cdot \sin^{2}}\frac{\delta}{2}}} \right)}}} & {{Eq}.\quad 17} \\{P_{3} = {{E_{3} \cdot E_{3}^{*}} = {\frac{\beta^{2}}{2}\left( {{\cos^{2}{\varphi \cdot \cos^{2}}\frac{\delta}{2}} + {\sin^{2}{\varphi \cdot \sin^{2}}\frac{\delta}{2}}} \right)}}} & {{Eq}.\quad 18} \\{P_{4} = {{E_{4} \cdot E_{4}^{*}} = {\frac{\beta^{2}}{2}\left( {{\sin^{2}{\varphi \cdot \cos^{2}}\frac{\delta}{2}} + {\cos^{2}{\varphi \cdot \sin^{2}}\frac{\delta}{2}}} \right)}}} & {{Eq}.\quad 19} \\{P_{12} = {{P_{1} + P_{2}} = \frac{\alpha^{2}}{2}}} & {{Eq}.\quad 20} \\{P_{34} = {{P_{3} + P_{4}} = \frac{\beta^{2}}{2}}} & {{Eq}.\quad 21} \\{P = {{P_{1} + P_{2} + P_{3} + P_{4}} = {\frac{1}{2}\left( {\alpha^{2} + \beta^{2}} \right)}}} & {{Eq}.\quad 22}\end{matrix}$

[0073] Using Jones vector notation representative of the polarizationstate of the light, it is possible to express an incident light E₀ in anarbitrary completely polarization state as shown in Equation (9). Afirst component of Equation (9) represents a scalar value of a Xdirectional component, and a second component of Equation (9) representsa scalar value of a Y directional component. In Equation (9), “f”represents a frequency, “δ₀” represents an initial phase, “δ” representsa phase difference between the X direction component and the Ydirectional component, and “φ” represents an azimuth angle.

[0074] When the incident light L4 represented by Equation (9) passesthrough the depolarizer 1 which is positioned as shown in FIG. 3, theincident light is separated into four light rays L7, L8, L9 and L10 asshown in FIG. 12. Then, the four light rays L7, L8, L9 and L10 passthrough the diffraction grating 5. Thereafter, the light rays L7, L8, L9and L10 come into four focal points F6, F8, F7 and F9 on the outgoingslit 7 a as shown in FIG. 8. At the time, the light rays L7, L8, L9 andL10 come into the focal points F6, F8, F7 and F9, respectively.

[0075] As the inclined angle of the inclined surface of the crystalplate 1 b becomes small, the distance between the focal points F6 and F7and the distance between the focal points F8 and F9 become small.Further, as the inclined angle of the inclined surface of the crystalplate 1 c becomes small, the distance between F6 and F8 and the distancebetween F7 and f9 becomes small. When the inclined angle of the inclinedsurface of the crystal plate 1 b is smaller than the inclined angle ofthe inclined surface of the crystal plate 1 c, the distance between thefocal points F6 and F7 and the distance between the focal points F8 andF9 becomes smaller than the distance between the focal points F6 and F8and the distance between the focal points F7 and F9.

[0076] “E1” of Equation (12) represents the state of the light ray atthe focal point F6, and “P1” of Equation (16) represents the power ofthe light ray at the focal point F6. “E2” of Equation (13) representsthe state of the light ray at the focal point F7, and “P2” of Equation(17) represents the power of the light ray at the focal point F7. “E3”of Equation (14) represents the state of the light ray at the focalpoint F8, and “P3” of Equation (18) represents the power of the lightray at the focal point F8. “E4” of Equation (15) represents the state ofthe light ray at the focal point F9, and “P4” of Equation (19)represents the power of the light ray at the focal point F9. “P₇₄ ” ofEquation (10) represents a partial polarizer of an azimuth angle θ. “G”of Equation (11) represents a diffraction grating whose diffractiveefficiency is equal to α of the X directional component and whosediffractive efficiency is equal to β of the Y directional component. “*”represents a complex conjugate in each of Equations (16) to (19).

[0077] In general, the diffraction grating 5 has the diffractiveefficiency which changes according to the oscillation direction of thelight which is incident thereon.

[0078] As readily understood from Equations (16) to (19), when thediffractive efficiency in X direction is “α=1” and the diffractiveefficiency in Y direction is “β=0”, only two focal points F6 and F7 areformed on the outgoing slit 7 a as shown in FIG. 9. As a result, a pairof light rays L7 and L9 is selected from the pair of light rays L7 andL9 and a pair of light rays L8 and L10. On the other hand, when thediffractive efficiency in X direction is “α=0” and the diffractiveefficiency in Y direction is “β=1”, because only two focal points F8 andF9 are formed on the outgoing slit 7 a, the pair of light rays L8 andL10 is selected from the pair of light rays L7 and L9 and the pair oflight rays L8 and L10.

[0079] As described above, when the inclined angle of the inclinedsurface of the crystal plate 1 b is comparatively small, the distancebetween the foal point F6 and the focal point F7 is small on theoutgoing slit 7 a. As a result, it is possible to regard the focal pointF6 and F7 as one focal point according to the characteristic of thespectroscope. As shown in Equation (20), the total of the power of thelight ray at the focal point F6 and the power of the light ray at thefocal point F7 is constant. That is, the light having the constant powercomes into one focal point (spot) in the arbitrary polarization state.

[0080] As a result, the spectrum as shown in FIG. 13B is displayed onthe spectrum display unit 10 shown in FIG. 3. It is possible to obtainthe spectrum having the stable central wavelength in the arbitrarypolarization state. When the signal processing unit 9 shown in FIG. 3has an adjusting function of always subtracting the constant Δλ from themeasured central wavelength, it is possible to measure the spectrumhaving the true central wavelength.

[0081] In other words, it is possible to measure the spectrum having thetrue central wavelength with respect to the incident light in thearbitrary polarization state. As a result, it is possible to improve thespectroscope in the spectroscopic characteristic thereof, in comparisonto the spectroscope which uses the conventional depolarizer.

Second Embodiment

[0082] Next, a second embodiment of the present invention will beexplained, as follows. Although the first embodiment has been explainedwith the outgoing slit 7 a wherein the four light rays L7, L8, L9 andL10 pass through the focal points F6, F8, F7 and F9 as shown in FIGS. 8and 9, the present second embodiment will be explained with an outgoingslit 7 b as shown in FIG. 10 instead of the outgoing slit 7 a.

[0083] The rectangular opening of the outgoing slit 7 b is a slit thatthe rectangular opening of the outgoing slit 7 a is shortened in thedirection which has nothing to do with a wavelength selection. As shownin FIG. 10, the outgoing slit 7 b cuts off two light rays L8 and L10which travel to the focal points F8 and F9, respectively, and allows twolight rays L7 and L9 which travel to the focal points F6 and F7 to passtherethrough. Therefore, because the two light rays L7 and L9 areselected as described above, it is unnecessary to relatively strengthenthe powers of the light rays L7 and L9, and to relatively weaken thepowers of the light rays L8 and L10, although it is necessary torelatively strengthen the powers of the light rays L7 and L9, and torelatively weaken the powers of the light rays L8 and L10 in the firstembodiment.

[0084] Like the first embodiment, if the inclined angle of the inclinedsurface of the crystal plate 1 b is small, the distance between thefocal points F6 and F7 becomes small on the outgoing slit 7 b. As aresult, it is possible to regard the focal points F6 and F7 as one focalpoint according to the characteristic of the spectroscope. As understoodfrom Equation (20), the total of the power of the light ray L7 at thefocal point F6 and the power of the light ray L9 at the focal point F7shown in FIG. 8, is constant. That is, it is possible to obtain onefocal point (spot) at which the light has a constant power in thearbitrary polarization state.

[0085] As a result, the spectrum as shown in FIG. 13B is displayed onthe spectrum display unit 10 shown in FIG. 3. It is possible to obtainthe spectrum having the stable central wavelength in the arbitrarypolarization state. When the signal processing unit 9 shown in FIG. 3has an adjusting function of always subtracting the constant Δλ from themeasured central wavelength, it is possible to measure the spectrumhaving the true central wavelength.

Third Embodiment

[0086] Next, a third embodiment of the present invention will beexplained, as follows. Although the first embodiment has been explainedwith the single path spectroscope which uses the diffraction gratingshown in FIG. 3 at one time, the third embodiment will be explained witha multi path spectroscope which uses the diffraction grating at two ormore times.

[0087] In other words, according to the first embodiment, the singlepath spectroscope is adopted so that the light passes through thediffraction grating which is the spectroscopic device, at one time.However, according to the third embodiment, the multi path spectroscopeis adopted so that the light passes through the diffraction gratingwhich is the spectroscopic device, at n times, and the depolarizer 1 isused in the spectroscope. As described above, when the depolarizer 1 isused in the multi path spectroscope, it is possible to obtain thefollowing remarkable effects.

[0088] According to the third embodiment, “α” is replaced with “α^(n)”,and “β” is replaced with “β^(n)”, in Equations (11) to (22). As thenumber n that the light passes through the diffraction grating becomesgreat, the powers of the light rays at two focal points F6 and F7 becomegreat relatively. Accordingly, it is easier to obtain only two focalpoints F6 and F7 as shown in FIG. 9. In other words, it is easier toselect any of light rays. As a result, it is possible to measure thecentral wavelength of the spectrum at a higher accuracy. On the otherhand, it is generally known that when the light passes through thediffraction grating at two or more times, the resolution of thewavelength is enhanced. As a result, when the depolarizer 1 of thepresent invention is used in the multi path spectroscope according tothe present third embodiment, it is possible to realize the spectroscopehaving a high wavelength resolution and a high wavelength accuracy.

[0089] According to the present invention, the following effect will beindicated.

[0090] As described above, when the depolarizer of the present inventionis used in the spectroscope, it is possible to measure the spectrumhaving the true central wavelength with respect to the incident light inthe arbitrary polarization state, and to realize the spectroscopewithout a polarization dependence of the spectroscopic device. In otherwords, it is possible to improve the spectroscope in the spectroscopiccharacteristic, in comparison to the spectroscope which uses theconventional depolarizer.

[0091] The entire disclosure of Japanese Patent Application No. Tokugan2001-196745 filed on Jun. 28, 2001 including specification, claims,drawings and summary are incorporated herein by reference in itsentirety.

What is claimed is:
 1. A depolarizer comprising: a first plate athickness of which continuously changes in a direction of 45 degreeswith a first optical axis of the first plate; and a second plate athickness of which continuously changes, and which is stuck on the firstplate; wherein an angle between the first optical axis of the firstplate and a second optical axis of the second plate, is 45 degrees, anda first reduction direction of the thickness of the first plate and asecond reduction direction of the thickness of the second plate isopposite to each other.
 2. The depolarizer as claimed in claim 1,comprising: a third plate a thickness of which continuously changes in adirection of 45 degrees with a third optical axis of the third plate,and which is stuck on the first plate; wherein the first optical axis ofthe first plate and the third optical axis of the third plate intersectorthogonally with each other, and the first reduction direction of thethickness of the first plate and a third reduction direction of thethickness of the third plate is opposite to each other.
 3. Thedepolarizer as claimed in claim 1, comprising: a fourth plate athickness of which continuously changes, and which is stuck on thesecond plate; wherein the second optical axis of the second plate and afourth optical axis of the fourth plate intersect orthogonally with eachother, and the second reduction direction of the thickness of the secondplate and a fourth reduction direction of the thickness of the fourthplate is opposite to each other.
 4. The depolarizer as claimed in claim2, comprising: a fourth plate a thickness of which continuously changes,and which is stuck on the second plate; wherein the second optical axisof the second plate and a fourth optical axis of the fourth plateintersect orthogonally with each other, and the second reductiondirection of the thickness of the second plate and a fourth reductiondirection of the thickness of the fourth plate is opposite to eachother.
 5. The depolarizer as claimed in claim 1, wherein each of thefirst plate and the second plate is made of any one of a crystal, acalcite, a mica and a magnesium fluoride.
 6. The depolarizer as claimedin claim 2, wherein the third plate is made of any one of a crystal, acalcite, a mica and a magnesium fluoride.
 7. The depolarizer as claimedin claim 3, wherein the fourth plate is made of any one of a crystal, acalcite, a mica and a magnesium fluoride.
 8. A spectroscope comprising:a depolarizer comprising: a first plate a thickness of whichcontinuously changes in a direction of 45 degrees with a first opticalaxis of the first plate; and a second plate a thickness of whichcontinuously changes, and which is stuck on the first plate; wherein anangle between the first optical axis of the first plate and a secondoptical axis of the second plate, is 45 degrees, and a first reductiondirection of the thickness of the first plate and a second reductiondirection of the thickness of the second plate is opposite to eachother; and a spectroscopic device a dispersion direction of whichintersects orthogonally with the first reduction direction of thethickness of the first plate.
 9. The spectroscope as claimed in claim 8,wherein the depolarizer comprising: a third plate a thickness of whichcontinuously changes in a direction of 45 degrees with a third opticalaxis of the third plate, and which is stuck on the first plate, thefirst optical axis of the first plate and the third optical axis of thethird plate intersect orthogonally with each other, and the firstreduction direction of the thickness of the first plate and a thirdreduction direction of the thickness of the third plate is opposite toeach other.
 10. The spectroscope as claimed in claim 8, wherein thedepolarizer comprising: a fourth plate a thickness of which continuouslychanges, and which is stuck on the second plate, the second optical axisof the second plate and a fourth optical axis of the fourth plateintersect orthogonally with each other, and the second reductiondirection of the thickness of the second plate and a fourth reductiondirection of the thickness of the fourth plate is opposite to eachother.
 11. The spectroscope as claimed in claim 9, wherein thedepolarizer comprising: a fourth plate a thickness of which continuouslychanges, and which is stuck on the second plate, the second optical axisof the second plate and a fourth optical axis of the fourth plateintersect orthogonally with each other, and the second reductiondirection of the thickness of the second plate and a fourth reductiondirection of the thickness of the fourth plate is opposite to eachother.
 12. The spectroscope as claimed in claim 8, wherein when a firstexternal surface opposite to a first stuck surface of the first plate,which is stuck on a second stuck surface of the second plate, and asecond external surface opposite to the second stuck surface, areinclined to an incident light, the incident light is refracted on thefirst external surface of the first plate and separated into a primaryfirst refracted light and a primary second refracted light, the primaryfirst refracted light is refracted on the second external surface of thesecond plate and separated into a secondary first refracted light and asecondary second refracted light, and the primary second refracted lightis refracted on the second external surface of the second plate andseparated into a secondary third refracted light and a secondary fourthrefracted light, a focal point of the secondary first refracted lightand a focal point of the secondary third refracted light are closer toeach other than the focal point of the secondary first refracted lightand a focal point of the secondary second refracted light, on anoutgoing slit for wavelength-selecting any of the secondary first,second, third and fourth refracted lights, and any one pair of a pair ofthe secondary first refracted light and the secondary third refractedlight and a pair of the secondary second refracted light and thesecondary fourth refracted light is selected.
 13. The spectroscope asclaimed in claim 12, wherein any one pair of the pair of the secondaryfirst refracted light and the secondary third refracted light and thepair of the secondary second refracted light and the secondary fourthrefracted light passes through an opening of the outgoing slit, and theother pair of the pair of the secondary first refracted light and thesecondary third refracted light and the pair of the secondary secondrefracted light and the secondary fourth refracted light is cut off bythe outgoing slit.
 14. The spectroscope as claimed in claim 8, wherein alight passes through the spectroscopic device at n times.